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Faidherbia - Africa's fertiliser factory

Lying in the heart of Africa, Malawi is well known for its export of tea, sugar and tobacco, which take up much of its arable land, but the countrys biggest agricultural export originates not from these commercial plantations; nor is it an intended one

It is soil, eroded from the fields of smallholder farms that support 80 per cent of Malawi's population.


Loss of Malawi's valuable soil can be seen by the extensive gully erosion visible on hillside plots, where heavy rains have breached planting ridges, and the rivers flow muddy brown with silt. As a result of declining fertility and degraded soil, crops often fail or yields are low and farmers struggle to feed their families. With the impact of global warming, farmers have "never been less prepared," asserts Peter Aagaard, director of the Conservation Farming Unit (CFU), which is based in Zambia but has an increasing following in Malawi.

The solution, according to the CFU, lies in minimising soil disturbance, while boosting fertility through planting of the nitrogen-fixing acacia tree, Faidherbia albida. This is in marked contrast to the traditional practice of land preparation in Malawi, where splitting and reforming ridges moves an estimated 700 million tons of soil every year, and results in high labour costs as well as water logging and soil degradation. Instead, farmers are being encouraged to dig small planting pits, a minimum tillage technique which disturbs only ten per cent of the soil. With funding from the Norwegian Government, the CFU is also training farmers to practice crop rotation, and to protect their soils by leaving crop residues on the ground instead of burning them.

Restoring tired soil

It may be counter-intuitive for farmers, but instead of ploughing a whole field, farmers are taught at field days and through farmer-led extension to dig small holes or "basins," about 25cm deep, 25cm wide and a metre apart. The holes are filled with compost or a small amount of fertiliser, and maize is rotated yearly with nitrogen-fixing legumes such as groundnuts, to prevent disease and soil exhaustion. Farmers with oxen can practise minimum tillage 'rip-and-furrow', where furrows carved approximately 25cm deep, 30cm wide and 90cm apart keep soil disturbance to a minimum.

With the planting of F. albida every ten rows, conservation agriculture is complete. Unusually, the tree remains leafy during the dry season, thereby offering the crop some protection from intense sun, but sheds its leaves during the rains. With trees planted at the correct density, the nutrient-rich leaves can supply the equivalent of 300 kg of fertiliser per hectare - a valuable asset for the many farmers who cannot afford to buy it. Research in Malawi and Zambia shows that mature trees can sustain unfertilised maize yields of 2.5 to 4 tons per hectare - 200 to 400 per cent more than national averages.

This system has another vital advantage. For farmers who plough their land, land preparation is traditionally done with the first rains, because of the need for moist soil. Not needing to plough means farmers no longer have to wait for oxen to be available, in order to plant their crop. As a farmer himself, Aagaard can empathise: "For every day we are planting late, we are losing 1.5 per cent of potential yield." Ploughing is also expensive at US$50 per hectare and often leads to soil compaction - resulting in reduced soil penetration.

Trees for life

The CFU has been promoting conservation agriculture since 1996 and runs 900 field days per year in order to convince farmers that increased yields are possible. Persuading farmers to stop ploughing, however, has been a challenge; "For many farmers," notes Aagaard, "ploughing is farming and farming is ploughing." In addition, since Faidherbia can take up to six years before it is fully mature, investing in tree planting is difficult, especially where formal land tenure is not secure.

Head of outreach services at the Zambia National Farmers Union, Hamusimbi Coillard, comments: "When someone is confronted with food insecurity, they need something immediate. This is not an instant thing." However in Zambia, more than 150,000 farmers have begun to practise conservation agriculture, which is now embedded in national policy.

Meanwhile, at the World Agroforestry Centre in Nairobi, director general Dennis Garrity is excited about the carbon sequestration potential of planting Faidherbia trees, in addition to the soil fertility benefits. "This is no panacea," he says, "but if this vision was applied on 50 million hectares of crop land, think of the carbon sequestration opportunities in Africa."

Georgina Smith